The invention pertains to encoding systems usable with video signals.
In pay or cable television systems, some form of signal scrambling is required to insure that only paid subscribers will receive the product. The use of signal scrambling also requires the use of receiver decoders or descramblers. The potential number of subscribers can be quite large. As a result, it becomes profitable for unauthorized organizations or companies to sell or provide unauthorized decoders or descramblers. For this reason, the scrambling system must be secure, not only against reception by normal televisions, but also against the design of inexpensive pirate receiver decoders or descramblers.
Existing devices are known for encoding and decoding video signals using line-rotation video scrambling. Line-rotation video scrambling intentionally interchanges segments of each video line prior to transmission. At the transmitting end, an encoder must be used accomplish this exchange.
At the transmission end of a conventional line-rotation video scrambling system, horizontal scan lines are cut at varying points and the line segments switched in time. This, in effect, rotates the end of each line to the beginning of the same line. At the reception end, the scrambled video lines are "rotated" back into the original time sequence by the local decoder apparatus.
The known encoder circuitry usable with linerotation scrambling requires the use of two memory circuits with information alternately read out of each. Each of the memory circuits is used to temporarily store a next scrambled line of video while the other memory is reading out a current, unscrambled line of video.
Line-rotation video encoding circuitry is often used in combination with other television transmitter circuitry. Since it is desirable to integrate as many circuits as possible onto a single chip, there exists a need to minimize the complexity of the encoder circuit without impairing its ability to carry out the encoding operation.